A. Field of the Invention
This invention relates to coal tar emulsion compositions for coating bituminous pavement and, more particularly, to the dilution of such compositions with water.
B. Description of the Prior Art
Bituminous surfaces, for example, roads, driveways and the like, are attacked by spilled lubricants and fuels. Aqueous tar emulsions for protecting such surfaces are available from several manufacturers. Normally, tar and water are not compatible. The emulsion of the tar in an aqueous medium, however, may be accomplished with the aid of an emulsifying or dispersing agent such as an organic soap or detergent and/or inorganic colloid such as a particulate clay-like bentonite, or, as U.S. Pat. No. 3,027,342 discloses, by incorporating a copolymer of acrylonitrile and butadiene into a coal tar composition and emulsifying it in an aqueous medium without the aid of an emulsifying agent.
The general requirements for emulsion of such type are set forth in the Federal Specification R-P-355d, "Coal Tar Pitch Emulsion." The chemical requirements are:
______________________________________ Maximum Minimum ______________________________________ Water, percent 53 -- Nonvolatile, percent -- 47 Ash of nonvalitiles, percent 40 30 Solubility of nonvolatiles in CS.sub.2, percent -- 20 Specific gravity 25.degree. C./25.degree. C. -- 1.20 ______________________________________
The physical requirements are that the emulsion is to be homogenous and its components are to show no separation or coagulaton that cannot be overcome by moderate stirring. The emulsion is to be capable of application and complete coverage of a bituminous pavement when applied by squeegee, brush, or other conventional mechanical methods to the surface of the pavement at a spreading rate of 1.5 to 2.5 gallons per 100 square feet in two coats. The tar component of the emulsion is to be a straight run high termperature coke oven tar conforming to requirements of Federal Specification R-T-143. Petroleum tar and oil and water gas tars are not to be used in such coatings even though they comply with R-T-143 because these materials are also attacked by petroleum products such as fuels.
Upon application, the emulsion normally dries in eight hours or less to yield a continuous dry film which protects the bituminous surface from the deteriorating effects of weather, of chemicals used for cleaning and deicing, and of grease, oil and gas spills. Without such protective film, sunlight also has a destructive effect on bituminous pavement, "drying out" or breaking down the binder that holds the stone aggregate together and causing the binding oils to oxidize; the bituminous pavement will become increasingly brittle and eventually shrink and crack. Then, frost damage may occur; water enters through such cracks, expands on freezing and exerts internal pressures to enhance this deterioration. The petroleum derivatives such as fuel oil, gasoline, and lubricating oils, when leaked or spilled, act as solvents and penetrate unprotected asphalt surfaces, thereby causing soft spots and further pavement deterioration. The dry protective film that results from the coal tar emulsion, being black and velvety in appearance, adds an aesthetic value to the pavement while it is providing a tight smooth, non-porous flexible film that resists deteriorating effects of weathering, freeze-thaw, oxidation, jet fuel, gasoline, kerosene and similar petroleum products. Such coal tar emulsions are intended to be used as a protective coat and not as a repair, leveling material or wearing surface, although such coatings generally exhibit excellent wearing properties.
Attempts have been made to vary the general characteristics of such coatings. U.S. Pat. Nos. 3,835,117 and 3,897,380 suggest adding to the emulsion a high amount, 10-20 pounds, of aggregate filler material such as coarse sand to each gallon of emulsion to provide greater strength and durability to the resulting film coatings; but then a special acrylonitrile/butadiene latex must be used to inhibit an undesirable "floating out" or separation of the sand from the film. This special acrylonitrile/butadiene latex is a latex which has an extraordinarily small particle size of between 400 and 1,000 angstroms; conventional acrylonitrile/butadiene latex has a particle size of about 0.25 microns. The special acrylonitrile/butadiene copolymer portion of the special latex is to be present to the extent of 2.5 to 15 percent of the weight of the tar content of the emulsion.
The capability of the special acrylonitrile/butadiene latex to maintain a high quantity of sand in matrix-like suspension even where a higher than ordinary amount of water is employed is, according to U.S. Pat. Nos. 3,835,117 and 3,897,380, theorized to be due to the extraordinarily small acrylonitrile/butadiene particle size. As comparison, the tar particles of the original tar emulsion have a particle size of from about 3-10 microns and the acrylonitrile/butadiene particles of the special latex have a particle size of between 400 and 1000 angstroms. This difference in particle size is believed to permit a distribution of the latex particles throughout a pavement sealing composition so as to provide within the aqueous coal tar emulsion a nearly continuous additional rubber phase which imparts a further thixo-tropic nature to the pavement sealing composition and permits the composition to be applied as and dry to films that are usually 2-10 times thicker than would ordinarily be expected from the conventional tar emulsion compositions.
It is believed, in accordance with this invention and despite the disclosures of U.S. Pat. Nos. 3,835,117 and 3,897,380, that a close relationship does exist between the quantity of sand to be used in the coating and the quantity of coal tar emulsion to be used, and if the amount of sand to be used exceeds this close relationship, then an insufficient amount of the coal tar binder exists to cover the sand and adhere the sand particles to each other and to the bituminous pavement surface. This recommended ratio is from 1 to 6 pounds of clean, coarse, dampened sand per gallon of coal tar emulsion, with the preferred ratio being 2 to 4 pounds of sand per gallon of emulsion. It is also believed that the tar emulsion composition should not be applied to the bituminous pavement surface in a manner as to develop thick coats for the reason that thick coats do not cure readily and often result in pick-up and tracking as persons or vehicles cross the coating, even after the coating has had a reasonable time to cure. If thick final coatings be specified, it is believed the better practice to be that several thin coats of emulsion should be applied to the surface and permitted to dry or cure and subsequent coatings applied thereafter and dried until the final resulting coating of specified size or thickness is reached.
The convenient way to obtain a thin coating is to dilute the composition and apply the composition in the conventional manner. The problem presented, however, is that diluting the tar emulsion with water tends to diminish the thixo-tropic nature and the coating tends to run and the sand to separate. In accordance with this invention, coal tar emulsions to be used as bituminous pavement coatings may be diluted by an amount of water equal to one and one-half times the amount of emulsion and still maintain the same apparent thixo-tropic nature as if the emulsion had not been diluted. This enables operators to use the same equipment to apply the composition to the surface to a conventional uniform layer on the surface and yet, when the surface dries, it is a uniform thin layer for the surface; a plurality of such layers being more effective than one thick layer.